U.S. patent application Ser. No. 07/569,080 filed on Aug. 17, 1990, abandoned, and entitled Monolithic Accelerometer discloses an integrated circuit comprising a suspended microstructure acceleration sensor and conditioning and resolving circuitry on the same chip. The device disclosed therein can be fabricated by the process generally outlined therein but is more preferably fabricated using the improved process disclosed herein. The disclosure of that application is incorporated herein by reference. The sensor portion of the accelerometer disclosed in that application comprises a polysilicon bridge suspended above a substrate by a series of posts. The polysilicon bridge comprises a suspended longitudinal beam having a plurality of fingers extending transversely therefrom. For each finger extending from the beam, there is a corresponding stationary finger positioned parallel and close thereto. The bridge is electrically conductive and is charged to a different voltage than the stationary fingers. The polysilicon bridge is resilient such that, under accelerative force, the bridge, including the fingers, will move relative to the corresponding stationary fingers. The capacitance between the movable fingers and the stationary fingers is observed and resolved to determine the magnitude of the accelerative force to which the sensor is subjected.
Other suspended microstructures for sensing acceleration and other phenomena are disclosed in U.S. Pat. Nos. 4,711,128; 5,025,346 and 5,054,320.
The fabrication of wafers embodying suspended microstructures is difficult and usually produces a relatively low yield of acceptable chips. A particularly troublesome problem during fabrication is contact and sticking of the microstructure to the substrate. Also, surfaces of the suspended microstructure frequently stick to other surfaces of the suspended microstructure or to other surfaces on the chip during processing. It is extremely difficult to separate the polysilicon microstructure from the substrate, another polysilicon microstructure, or other components of the chip once it has come in contact therewith.
Liquid surface tension effects are among the most significant causes of the microstructure coming in contact with other objects, such as the substrate or other portions of the microstructure. Liquid surface tension effects occur, for instance, during drying after a wet etching step.
In fabricating suspended microstructures, a layer of material (from which the microstructure is to be constructed) is typically deposited over a previously deposited sacrificial layer and then etched into the desired form. The sacrificial layer is then removed by a wet etching process in which the wafer is exposed to a chemical etching solution which dissolves the sacrificial layer but does not affect the material from which the microstructure is formed, e.g., polysilicon. The wafer is then washed in a rinse fluid. As the rinse fluid is removed, the surface tension of the liquid exerts forces on the delicately suspended microstructure, tending to pull the microstructure into contact with the substrate or with other portions of the microstructure or other components of the circuit. A combination of various forces, including adhesive forces and electrostatic forces, makes it extremely difficult to separate the contacting portions. Electrostatic forces may also contribute to the initial attraction of the microstructure to the other surfaces, leading to contact. Accordingly, when undesirable contact occurs, typically the chip is irreparable and must be discarded.
The sacrificial layer must be able to withstand high temperatures on the order of 1000.degree. C. and higher) in order to withstand the temperatures involved in the deposition of the structural layer and subsequent integrated circuit processing steps in conventional fabrication techniques. Accordingly, the sacrificial layer typically must be composed of a material, such as a low temperature, chemical vapor deposited oxide, which can only be practically removed by a wet etching processing. A sacrificial layer of photoresist material, for instance, could not withstand the temperatures involved in polysilicon deposition, i.e., about 600.degree. C. Further, dry etching to remove the sacrificial layer would eliminate the surface tension problem but is typically not possible because a dry etch process would likely damage the suspended microstructure material, e.g., polysilicon, due to the low selectivity of such etching techniques.
Several researchers have reported on this problem and proposed methods to eliminate the effects of liquid surface tension. For instance, H. Guckel, J. J. Sniegowski, T. R. Christenson and F. Raissi, "The Application of Fine Grained, Tensile Polysilicon to Mechanically Resonant Transducers", Sensors and Actuators, A21, 1990, pp. 346-351, suggest a method in which the wet etchant final rinse fluid is frozen and sublimated to avoid the deleterious effects of liquid surface tension on microstructures. The disclosed method requires the transfer of wet wafers into a refrigeration unit to freeze the fluid (a water/methanol mixture). The wafer is then placed in a vacuum system to sublimate the frozen fluid. The technique requires special chemicals and the direct transfer of wet wafers into refrigeration and vacuum system. Further, sublimation of the frozen fluid requires very long times, on the order of hours. The proposed system is not practical for manufacturing.
Accordingly, it is an object of the present invention to provide a high yield method of fabricating integrated circuits comprising suspended microstructures.
It is a further object of the present invention to provide an improved method for fabricating suspended microstructures on integrated circuit wafers.